Power plants, water purification plants, and oil refineries that are lifelines as a base of social life, and power transmission and distribution systems, water distribution systems and transportation systems associated therewith are required to be worldwide enhanced with the population growth and the improvement of a standard of living. In order to cope with such a situation, various equipment that becomes the lifeline becomes large in size, and the number of power generation equipment, water purification equipment, oil refining equipment, substation equipment, water distribution equipment, or pipelines steadily increases in individual systems or individual plant. For efficient operation of the system or the plant and ensuring of safety, a need for technology for coupling individual equipment to a network to recognize a situation of each equipment or control each equipment has been advocated. In building such a network, wired technology and wireless technology are applicable and have respective characteristics, and accordingly, both are used selectively or in combination depending on a function or performance required by each system and each plant.
Since a network to which wireless technology is applied, that is, a wireless network does not require hardware such as wires between respective equipment, there are advantages that it is easy to introduce the wireless network and a cost of installation is low. Since a plurality of transmission paths of the wireless network are automatically formed from a transmission end to a reception end in a free space by electromagnetic waves, the wireless network is advantageous to improve reliability of communication based on transmission path multiplexing. Further, since a specific transmission path is invisible, it is difficult to specify the transmission path, and a possibility that an entire system is disabled due to blocking of the transmission path by an outsider is significantly lower as compared to a network to which wired technology is applied. On the other hand, since the transmission path is open to outsiders, concealment of information through the transmission path and detection of an action of the outsiders tampering a specific transmission path is generally difficult as compared to the network to which the wired technology is applied. Therefore, it is necessary to improve confidentiality using an encrypting technology or the like.
When a wireless network is applied to a system or a plant that becomes a lifeline, a frequency of electromagnetic waves that can efficiently propagate through a free space is limited to approximately 300 MHz to 3 GHz, and accordingly, dimensions of various equipment are several times or more greater than a wavelength of electromagnetic waves used by the wireless network. Therefore, the electromagnetic waves are reflected by the equipment, and a plurality of transmission paths are formed by a plurality of reflections between the reception end and the transmission end. The electromagnetic waves are transverse waves and have a vector component called a polarization in a direction orthogonal to a traveling direction. When the electromagnetic waves are reflected, a direction of polarization varies depending on an angle of incidence, and a phase of the electromagnetic waves varies depending on a path length of electromagnetic waves generated in a transmission end and a reception end by a plurality of reflected waves. Accordingly, there is a problem in that a vector sum of a plurality of electromagnetic waves arriving at the reception end is significantly reduced and quality of communication between transmission and reception is degraded.
As technologies for solving such problems, technologies described in PTL 1 to PTL 4 are known.
In the technology described in PTL 1, a plurality of rotation polarizations having different rotational speeds modulated with the same signal are transmitted. Accordingly, a situation of a vector sum of a plurality of electromagnetic waves arriving at a reception end is changed, and degradation of quality of communication between transmission and reception is prevented.
In the technology described in PTL 2, quadrature modulation is performed on transmission waves so as to change a modulation scheme according to a percentage of orthogonal components of the electromagnetic waves arriving at a reception end. That is, when a vector sum of the electromagnetic waves in the reception end is reduced, an amount of information transmitted from a transmission end is increased so as to prevent degradation of communication quality.
In the technology described in PTL 3, a polarization of transmission waves is rotated, and a rotation frequency is changed and selected so that a vector sum of a plurality of electromagnetic waves arriving at a reception end is not reduced. Accordingly, degradation of quality of communication between transmission and reception is prevented.
In the technology described in PTL 4, a beam of a strong directivity antenna is mechanically rotated using the strong directivity antenna so as to change a situation of a vector sum of a plurality of electromagnetic waves arriving at a reception end. Accordingly, degradation of quality of communication between transmission and reception is prevented.